Combustible materials useful as propellants or gas-generants are provided for a variety of applications. These applications include, but are not limited to, in the case of propellants, supplying the propulsive energy required to propel a rocket and, in the case of gas generants, supplying sufficient combustion gases, for example, to inflate a vehicle air-bag in the event of a collision. The propellant or gas-generant is typically located within a protective outer case.
While the propellant or gas-generant is maintained in storage or being transported, for example, prior to deployment of a rocket or while an air-bag remains undeployed, it is critically important to avoid where possible, or to at least minimize the effects of, unintended combustion and/or explosion of the combustible material. Prior to deployment, the propellant and/or the gas-generant composition within the case may be exposed to a variety of stimuli such as to extremes of temperature from fire or other localized or dispersed heat sources, varying humidity levels, exposure to incoming artillery including shrapnel, bullets and the like in a variety of environments wherein, e.g., rockets and/or air-bags have utility. In response to such stimuli, due to the explosive nature of these compositions, unintended combustion and explosion of propellants and/or gas-generants can occur causing catastrophic damage to life and/or to property.
Most often, just prior to explosion, the case in which a propellant and/or a gas-generant composition is located becomes highly pressurized. Such pressurization induces the case to violently explode, causing potentially catastrophic consequences. If the case itself or any substantial part thereof is enclosed within or substantially encloses materials such as metals, alloys and the like that may yield shrapnel, the extent of damage caused by the explosive force of the propellant and/or gas-generant within the case is exponentially increased.
To avoid or at least minimize the consequences of unintended combustion and explosion of the combustible material, it is desirable to provide a case that will not violently explode in response to exposure of the material therein to a variety of unforseen stimuli. Thus, there is a need to provide a case which minimizes the tendency to pressurize and then explode. Such a case, in the event of an explosion, would minimize or preferably eliminate the tendency to yield shrapnel while retaining its dimensional stability. Dimensional stability, as used herein, refers to a case having sufficient hoop and longitudinal load for either propulsive and/or gas-generant applications and is further defined below. Such a case would provide the advantage of minimizing or altogether eliminating the potential danger, for example, from an exploding rocket while in storage or in transport or from a vehicle air-bag whose deployment is initiated by an intervening cause such as a fire involving the vehicle in which the air bag is installed.
One approach to providing such a case is to form one which is permeable to combustion gases when the case is subjected to temperatures above the ambient temperature but below the auto-ignition temperature of the combustible material, e.g., the propellant and/or gas-generant contained therein. It is further desirable to form the case from materials that eliminate or minimize the use of materials such as metals, various alloys, or other similar materials that may yield shrapnel upon explosion. It is, therefore, desirable to eliminate or at least minimize the inclusion of such materials either within the case itself, substantially surrounding the case or being substantially surrounded by the case. Avoidance of or the minimal use of such materials thus reduces the potential for and extent of damage to property or injury to persons in the vicinity of an explosion typically occurring under unintended or uncontrolled circumstances.
U.S. Pat. Nos.: 5,228,285; 5,060,470; and 5,369,955 are noted as examples of attempts to provide a case such as that described above. However, all of these patents disclose cases that are plagued by one or more critical 1s deficiencies. For example, in the '285 patent, a case comprising a slotted and capped metal sleeve overwrapped with fibers coated with a matrix resin is disclosed. The presence of the metal sleeve (see column 4, lines 25-27 of the '285 patent) in the barrel of the case is required to provide the necessary longitudinal load. The "longitudinal load", as the term implies, refers to the resistance provided by the case in response to the force longitudinally exerted thereon along its major longitudinal axis, L, upon controlled combustion of the combustible material. See FIG. 7, infra. The overwrap provides the necessary hoop load. The "hoop load" refers to the resistance provided by the case in response to the force radially exerted by controlled combustion of the combustible material thereon. Hoop forces are denoted by the radially directed arrows marked F in FIG. 7.
The term "dimensional stability" as used herein is defined as providing sufficient longitudinal and hoop loads for propulsive and/or gas-generating applications and the like. If sufficient hoop load and/or longitudinal load is not provided by the case, it will break up during the controlled combustion of the propellant and/or gas-generant contained therein to the degree that, for example, the rocket will not reach its target or the air-bag will not properly deploy. The case of the '285 patent, as noted above, requires the use of substantial amounts of metal in the barrel section which can form extremely destructive shrapnel upon explosion. As such, the case of '285 fails to fill the aforementioned need.
Likewise, the '470 patent relies on the use of memory metal members (item 26 of FIG. 2 depicted in the '470 patent) to provide a gas permeable case. Again, as with the '285 patent, the memory metal itself can form extremely destructive shrapnel upon explosion. As such, the case of the '470 patent also fails to fill the aforementioned need.
The '955 patent relies on the use of a case 12 (See FIG. 1 therein) comprising a fibrous material impregnated with a matrix material. The matrix material comprises first and second resin components. According to the '955 patent, the first resin component is any suitable resin which under actinic radiation conditions may be cured to immobilize the mass of the first resin without simultaneously curing the second resin component. The use of an actinic radiation curable material requires the use of cumbersome radiation curing equipment and further limits the choice of the first resin to one that is curable by exposure to such radiation and which cures under curing conditions different than those of the second resin. Use of such first and second resins and their peculiar curing requirements thus introduce unnecessary and undesirable limitations and complications in the formation of a case as described and sought.
Further, according to the '955 patent, the case described therein relies on the use of fibrous material that weakens as the melt point of the fiber is approached. See column 3, lines 30-43 of the '955 patent. Examples of such fibrous materials are polyolefin fibers (Spectra.RTM. 1000) or polyethylene fibers (Spectra.RTM. 900) as noted in the '955 patent at column 3, lines 46 and 56, respectively. Further, as also noted in the '955 patent, these fibers have melting points between 200.degree. F. to 300.degree. F., which are intermediate between the ambient temperature (up to 165.degree. F.) and the auto-ignition temperature (of 400.degree. F.) described therein. The use of such fibers, having melting points between 200.degree. F. to 300.degree. F., undesirably and disadvantageously limits the temperature at which the first and second resins may be cured. See column 4, lines 6-8 of the '955 patent. In particular, to maintain fiber integrity, the first and second resins cannot be cured above about 200.degree. F. Additionally, the low melting points in the range of 200-300.degree. F. of such fibers limit their use. For example, double base propellants have an auto-ignition temperature of only 320.degree. F. The close proximity of the auto-ignition temperature and the melting point temperatures of the fibers (of the '955 patent) prohibitively restricts their use to propellants and/or gas-generants having auto-ignition temperatures substantially higher than 320.degree. F. If high melting point-fibers could be used such limitations would be entirely avoided.
Further, U.S. Pat. No. 4,892,764 to Drain et al. discloses a filament reinforced resin composite article serving as a pressure vessel. See column 1, line 62 of the '764 patent. According to the '764 patent, the filament includes materials such as glasses, graphite, boron, Kevlar.RTM., polyacetal and more generally, wood, metal, natural and synthetic polymers, etc. See column 17, lines 35-38 of the '764 patent. Further according to the '764 patent, the resins employed in filament winding must exhibit constant viscosity and long pot life to avoid sagging and to maintain uniform thickness during the filament winding process. As noted at columns 3 and 4 of the subject patent, the resin compounds are first and second epoxy resins identical to those described in the '955 patent. Thus, the unnecessary complexity introduced by the use of actinic radiation curable resins previously noted with the '955 patent equally apply to the '764 patent.
The major design flaw of the articles made according to the '764 patent is that the '764 patent is directed to forming "pressure vessels". Such vessels, as the term implies, are directed to maintaining and containing any pressure build-up that may occur during the combustion of a propellant and/or a gas-generant. In effect, the articles of the '764 patent are directed to being impermeable to propellant and/or gas-generant combustion gases. In fact, no mention of an article having permeability to combustion gases produced at temperatures above ambient temperature but below the auto-ignition temperature of the combustible material is made in the '764 patent.
Thus, the need to provide a case which is substantially free of metal or similar materials capable of forming dangerous shrapnel, which uses fibers having melting points above about 300.degree. F., preferably, well above about 300.degree. F., which is permeable to combustion gases above ambient temperature but below the auto-ignition temperature of a propellant, gas-generant or other combustible material contained therein while retaining its dimensional stability while in use remains unfulfilled.